The purpose of these studies is to establish the basic developmental and cellular events operating in regulating the genes for fetal (gamma) and adult (beta A and beta C) globin in sheep erythroid cells. Induction of Hb C synthesis may be achieved in vitro or in vivo by exposure of erythroid stem cells to high erythropoietin (epo) concentration. The amount of Hb C synthesis appears to be related to the degree of differentiation of stem cells which give rise to erythroid colonies in vitro; the earliest stem cells give rise to colonies making more Hb C. We have now investigated the possibility of a second level of control by a cellular mechanism; namely, modulation of the amount of Hb C produced during terminal maturation of erythroblasts. A young adult animal was injected with epo and bone marrow samples obtained at intervals thereafter were fractionated into young, intermediate, and mature erythroblasts. There was a small increase in the proportion of Hb C produced during maturation of the bone marrow erythroblasts derived from stem cells committed by exposure to epo. In other experiments, an assay was devised to quantitate hemoglobin synthesis in individual fetal erythroid colonies in vitro. All colonies examined were shown to be making both Hb F and Hb C; a clonal or stem cell selection mechanism for regulation of Hb C was excluded.